The conclusion of the argument is: "many people are willing to pay a high price for just a little additional convenience."
Why? We know that microwave popcorn takes 3 minutes to pop, while conventional popcorn takes 7 minutes to pop. However, by weight, microwave popcorn costs over five times as much as conventional popcorn and is still very popular.
Answer choice (B) states: "Most people who prefer microwave popcorn do so because it is less fattening than popcorn that is popped conventionally in oil."
Answer choice (B) gives us another explanation as to why many people choose microwave popcorn over conventional popcorn other than just that microwave popcorn has "a little additional convenience," i.e. it is also less fattening than conventional popcorn.
Thus, if answer choice (B) were true, it would call into question the conclusion of the passage since it is no longer necessarily true that many people are wiling to pay a high price JUST for a little additional convenience (they're obviously also paying extra for it because it is less fattening).
Hope that was helpful! Please let us know if you have any other questions.
